diff --git a/src/wormhole/cli/runner.py b/src/wormhole/cli/runner.py
index ac984c1..8c373b1 100644
--- a/src/wormhole/cli/runner.py
+++ b/src/wormhole/cli/runner.py
@@ -4,7 +4,8 @@ start = time.time()
 import os, sys, textwrap
 from twisted.internet.defer import maybeDeferred
 from twisted.internet.task import react
-from ..errors import TransferError, WrongPasswordError, WelcomeError, Timeout
+from ..errors import (TransferError, WrongPasswordError, WelcomeError, Timeout,
+                      KeyFormatError)
 from ..timing import DebugTiming
 from .cli_args import parser
 top_import_finish = time.time()
@@ -49,7 +50,8 @@ def run(reactor, argv, cwd, stdout, stderr, executable=None):
     d.addBoth(_maybe_dump_timing)
     def _explain_error(f):
         # these errors don't print a traceback, just an explanation
-        f.trap(TransferError, WrongPasswordError, WelcomeError, Timeout)
+        f.trap(TransferError, WrongPasswordError, WelcomeError, Timeout,
+               KeyFormatError)
         if f.check(WrongPasswordError):
             msg = textwrap.fill("ERROR: " + textwrap.dedent(f.value.__doc__))
             print(msg, file=stderr)
@@ -58,6 +60,9 @@ def run(reactor, argv, cwd, stdout, stderr, executable=None):
             print(msg, file=stderr)
             print(file=stderr)
             print(str(f.value), file=stderr)
+        elif f.check(KeyFormatError):
+            msg = textwrap.fill("ERROR: " + textwrap.dedent(f.value.__doc__))
+            print(msg, file=stderr)
         else:
             print("ERROR:", f.value, file=stderr)
         raise SystemExit(1)
diff --git a/src/wormhole/errors.py b/src/wormhole/errors.py
index 08ebae7..47b9c2e 100644
--- a/src/wormhole/errors.py
+++ b/src/wormhole/errors.py
@@ -36,6 +36,13 @@ class WrongPasswordError(Exception):
     # or the data blob was corrupted, and that's why decrypt failed
     pass
 
+class KeyFormatError(Exception):
+    """
+    The key you entered contains spaces. Magic-wormhole expects keys to be
+    separated by dashes. Please reenter the key you were given separating the
+    words with dashes.
+    """
+
 class ReflectionAttack(Exception):
     """An attacker (or bug) reflected our outgoing message back to us."""
 
diff --git a/src/wormhole/test/test_wormhole.py b/src/wormhole/test/test_wormhole.py
index ba54123..795b6b6 100644
--- a/src/wormhole/test/test_wormhole.py
+++ b/src/wormhole/test/test_wormhole.py
@@ -7,7 +7,8 @@ from twisted.internet import reactor
 from twisted.internet.defer import Deferred, gatherResults, inlineCallbacks
 from .common import ServerBase
 from .. import wormhole
-from ..errors import WrongPasswordError, WelcomeError, UsageError
+from ..errors import (WrongPasswordError, WelcomeError, UsageError,
+                      KeyFormatError)
 from spake2 import SPAKE2_Symmetric
 from ..timing import DebugTiming
 from ..util import (bytes_to_dict, dict_to_bytes,
@@ -818,6 +819,23 @@ class Wormholes(ServerBase, unittest.TestCase):
         yield w2.close()
         self.flushLoggedErrors(WrongPasswordError)
 
+    @inlineCallbacks
+    def test_wrong_password_with_spaces(self):
+        w1 = wormhole.wormhole(APPID, self.relayurl, reactor)
+        w2 = wormhole.wormhole(APPID, self.relayurl, reactor)
+        code = yield w1.get_code()
+        code_no_dashes = code.replace('-', ' ')
+
+        with self.assertRaises(KeyFormatError) as ex:
+            w2.set_code(code_no_dashes)
+
+        expected_msg = "code (%s) contains spaces." % (code_no_dashes,)
+        self.assertEqual(expected_msg, str(ex.exception))
+
+        yield w1.close()
+        yield w2.close()
+        self.flushLoggedErrors(KeyFormatError)
+
     @inlineCallbacks
     def test_verifier(self):
         w1 = wormhole.wormhole(APPID, self.relayurl, reactor)
@@ -875,4 +893,3 @@ class Errors(ServerBase, unittest.TestCase):
         yield self.assertFailure(w.get_code(), UsageError)
         yield self.assertFailure(w.input_code(), UsageError)
         yield w.close()
-
diff --git a/src/wormhole/wormhole.py b/src/wormhole/wormhole.py
index fafb966..7392433 100644
--- a/src/wormhole/wormhole.py
+++ b/src/wormhole/wormhole.py
@@ -15,7 +15,7 @@ from . import __version__
 from . import codes
 #from .errors import ServerError, Timeout
 from .errors import (WrongPasswordError, UsageError, WelcomeError,
-                     WormholeClosedError)
+                     WormholeClosedError, KeyFormatError)
 from .timing import DebugTiming
 from .util import (to_bytes, bytes_to_hexstr, hexstr_to_bytes,
                    dict_to_bytes, bytes_to_dict)
@@ -476,6 +476,10 @@ class _Wormhole:
 
     def _event_learned_code(self, code):
         self._timing.add("code established")
+        # bail out early if the password contains spaces...
+        # this should raise a useful error
+        if ' ' in code:
+            raise KeyFormatError("code (%s) contains spaces." % code)
         self._code = code
         mo = re.search(r'^(\d+)-', code)
         if not mo: